Transient responses of superconducting lead films measured with picosecond laser pulses

Picosecond laser pulses are used, for the first time, to excite superconducting Pb films out of equilibrium. Pb microbridges are used to measure the threshold power needed to drive the Pb film normal as a function of temperature. Pb tunnel junctions are used to measure the quasiparticle relaxation times at temperatures close to $T_c$ and at low reduced temperatures. The measured quasiparticle relaxation time for a 500-Å Pb-$\mathrm{Pb}{\mathrm{O}}_x$-500-Å Pb tunnel junction is 3.1 nsec for the sample in vacuum at temperatures near $T_c$, and 2.5 nsec for the sample in superfluid at low reduced temperatures. At temperatures near $T_c$, the quasiparticle relaxation time for Pb is shown to be identical to the phonon escape time instead of the effective quasiparticle recombination time. At low reduced temperatures, the quasiparticle relaxation, for the case of excess quasiparticle density being much larger than the thermal equilibrium value, is shown to be approximately exponential with a time constant equal to twice the phonon escape time. The phonon transmissivities for the Pb-quartz, Pb-superfluid He, and Pb-$\mathrm{Pb}{\mathrm{O}}_x$ interfaces are determined from these relaxation time measurements to be 0.16, 0.16, and 0.2, respectively.